Many vehicles have a defogging and defrosting heater conductor on their rear windshields (glass panels). Such a heater conductor needs to be inspected to confirm that it is not disconnected when the vehicle is manufactured and assembled.
There has been proposed a method of inspecting a heater conductor by passing an electric current through the heater conductor under given conditions and determining whether the heater conductor is conductive or disconnected from a temperature distribution of a thermal image that is obtained when the rear windshield is imaged by an infrared camera (see, for example, Japanese Laid-Open Patent Publication No. 2003-215081 and Japanese Laid-Open Patent Publication No. 2004-109038). According to the inspecting method, when a wire disconnection is to be determined, the thermal image data produced by the infrared camera is compared with configurational data of a reference heater conductor on a two-dimensional basis to identify a disconnected location.
The inspecting method is directed to the inspection of only the rear windshield. Actually, after a rear windshield is mounted on a vehicle, it is necessary to provide a systematic quality guarantee covering electric parts that include a defroster switch, a harness, etc. Therefore, the heater conductor on the completed vehicle is inspected again for a wire disconnection on a final inspection line. On the inspection line, it is customary for the inspector to turn on the defroster switch and then, after a given time has elapsed, to directly touch the surface of the rear windshield to confirm a temperature rise thereof.
Since it takes some time for the heater conductor to become heated, the inspector needs to turn on the defroster switch prior to the inspection of the heater conductor. This process is burdensome for the inspector. If the inspector forgets to turn on the defroster switch or operates the defroster switch improperly, then the heater conductor remains to be at the normal temperature when it is to be inspected. At this time, the inspector has to turn on the switch and wait until the heater conductor becomes heated, which is an inefficient sequence of operation.
According to the method of comparing the thermal image data produced by the infrared camera with the configurational data of the reference heater conductor on the two-dimensional basis as disclosed in Japanese Laid-Open Patent Publication No. 2003-215081 and Japanese Laid-Open Patent Publication No. 2004-109038, it is difficult to automatically spot the disconnected location, and part of the process needs to be visually judged by the inspector.
According to the inspection for a wire disconnection using the infrared camera, since it takes some time for the heater conductor to become heated, the defroster switch may be turned on to heat the heater conductor in another inspection process prior to the inspection of the heater conductor, and the infrared camera may capture an image of the rear windshield when the vehicle arrives at the process of the inspection for a wire disconnection.
Depending on how the inspection processes are performed, the inspection line may progress at different speeds, and the time required from the other prior inspection process to the wire disconnection inspection process may suffer fluctuations. In addition, depending on the state of the battery connected to the heater conductor, the voltage supplied by the battery may differ, and the temperature rise of the heater conductor may not necessarily be constant. Consequently, according to the method of comparing the thermal image data produced by the infrared camera with the configurational data of the reference heater conductor on the two-dimensional basis as disclosed in Japanese Laid-Open Patent Publication No. 2003-215081 and Japanese Laid-Open Patent Publication No. 2004-109038, the data may be matched to different degrees, and no stable inspection may not be performed.
The temperature rise of the heater conductor tends to differ depending on the season and the temperature difference between morning and evening time zones even at a constant time after the defroster switch is turned on. Therefore, the inspection may be inaccurate when the thermal image data produced by the infrared camera is processed in a fixed uniform way.